Meltblowing is a process for manufacturing nonwoven products by extruding molten thermoplastic resins through fine capillary holes (orifices) and blowing hot air on each side of the extruded filaments to attenuate and draw down the filaments. The filaments are collected on a screen or other suitable collection device as a random entangled nonwoven web. The web may be withdrawn and further processed into consumer goods such as mats, fabrics, webbing, filters, battery separators, and the like. Also, the consumer goods may be produced in line with the meltblowing line.
As indicated above, the present invention relates to the post-treatment charging of nonwoven webs to alter the filament spacing and structure of the webs and to increase the filtering efficiency of the webs. It should be observed that the terms "filaments" or "fibers" are used interchangeably herein, although "fibers" in nonwovens generally refers to discontinuous strands and "filaments" as continuous strands. The present invention contemplate webs with continuous filaments and/or discontinuous fibers.
Since the development of the meltblowing process by the Naval Research Laboratory in 1951 (published in 1954 by the U.S. Department of Commerce in an article entitled "MANUFACTURE OF SUPERFINE ORGANIC FIBERS"), there has been a considerable effort by several companies operating in the industry to find new uses for the nonwoven product having microsized fibers. Because of the random, geometric assembly or structure of the fibers, and relatively small fiber size, the fibers have received extensive use as filters.
In the formation process for most random laid or unordered fibrous webs, the pore size that develops is inversely related to the square of the fiber diameter. The spunbonded process is distinguished from meltblowing by self-bonding and non uniform draw down (plastic deformation) of filaments forming the web. Thus meltblown webs have a relatively broad distribution of fiber diameters. Typical nonwoven webs produced by meltblowing have fiber diameters of 0.5 to 20 microns, preferably 0.5 to 8 microns, making them suitable for filtering out 5 micron particles at 80 percent efficiency or greater. It is known that filtration can be improved by practicing the web formation process to produce smaller and smaller diameter fibers while controlling other formation parameters such as porosity and thickness. As noted above, this results in smaller pore size thereby improving the efficiency of particle removal in filtration. By operating the meltblowing process under extreme conditions, the fiber size can be produced in the order of 0.1 to 5 microns. The process, however, has the following disadvantages: low production rates, high energy usage. In order to improve the properties of the nonwoven web, efforts have been made to post-treat the webs by a variety of processes. Such efforts have included post calendering the web to improve, the tensile strength of the web, post electrification as disclosed in U.S. Pat. No. 4,592,815 to improve filtration performance of the web, to name but two of such efforts. It is significant to note that none of these prior art techniques have been directed specifically at planar consolidation to reduce the size of the pores in the web.
Calendering of nonwovens flattens fibers and consolidates the web in a direction normal to the plane of the web and reduces the thickness. This, however, leads to reduction in permeability which is an important property to conserve for many filtration purposes. U.S. Pat. No. 4,048,364 discloses a process for drawing the meltblown web in the machine direction (MD) to produce a tenfold increase in the tensile strength of the post-drawn web. It is significant to note, however, that the precursor web required in the above invention contains relatively course fibers (10 to about 40 microns average fiber diameter) and polymer of low crystallinity. Low crystallinity generally means about 22% or less. The extensive drawing of the web reduces the diameter of the fibers in the machine direction to an average diameter of 1 to 8 microns at draw ratios ranging from 2:1 to 10:1 and preferably 5:1 to 7:1. The main purpose of the process is to increase the molecular orientation to enhance the strength of the greatly drawn fibers. Precursor webs of very high post processing draw ratio capability are required in order to prevent rupture of fibers in the web drawing process. Tests have shown that the stretching of a precursor web having hot (e.g., 10.degree. F. less than the melting point of the precursor web) drawing capabilities from about 5:1 to 10:1 does not alter the measure of pore size of the web. This is probably due to the fact that the high and easy drawability of the fibers prevents the development of sufficient, compressive forces to bend the stout fibers in the web and physically reduce its pore dimensions and measures of pore size distribution in general.
Many of the most recent uses for non-woven webs of fibrous materials involve the production of filter material. Most non-woven materials have structures such that there are many small pores in the surfaces of the webs which are connected to passageway through the thickness of the web. These pores and passageways are usually small enough to remove larger particulates from, for example, an air or other fluid flow. However, there is an increasing demand for filter material with increased ability to remover smaller and smaller particles from fluid flows.
Electrically charged fibrous materials to be used as a filtration medium have been known for some time. In U.S. Pat. No. 2,740,184, Thomas discloses a process of charging thermoplastic, fibrous webs by softening the fibers in the webs with heat and, while such fibers are soft, subjecting them to suitable electrostatic field to produce a charged web.
U.S. Pat. No. 4,215,682 to Kubik, et al., discloses methods for the preparation of electrically charged melt-blown fibers in which the melt-blown fibers are charged with an electrostatic charge immediately after they are formed and then deposited on a web. Similar hot charging processes are disclosed, for example, in U.S. Pat. No. 4,904,174 to Moosmayer, et al., and U.S. Pat. No. 5,122,048 to Deeds. Webs charged by such hot charging methods do not have the charge density that is necessary to remove the finest of particles from air flows or other fluid flows.
There are also several cold charging processes for the preparation of charged webs. For example, U.S. Pat. No. 4,375,718 to Wadsworth, et al., and U.S. Pat. No. 4,588,537 to Klaase, et al., describe processes for the corona charging of combined webs made from layers of materials with differing conductivities. U.S. Pat. No. 4,592,815 to Nakao describe placing a nonconductive web between the surface of a grounded metal electrode and a series of discharge electrodes. The cold charging methods also have problems developing the desired charge densities and, in addition, suffer from the added problem of having the charge bleed off the web with time.